If four vertices of a regular octagon are chosen at random,then the probability that the quadrilateral formed by them is a rectangle is

$A$ bag contains $6$ white and $4$ black balls. $A$ die is rolled once and the number of balls equal to the number obtained on the die are drawn from the bag at random. The probability that all the balls drawn are white is:

If three letters can be posted to any one of the $5$ different addresses,then the probability that the three letters are posted to exactly two addresses is:

$A$ bag contains $3$ white and $6$ red balls. Four balls are drawn at a time randomly. The probability of getting at least $2$ red balls is:

Three faces of a fair die are yellow,two faces are red and one face is blue. If the die is tossed $3$ times,then the probability that the colours yellow,red and blue appear is (need not be in that order).

Five numbers $x_{1}, x_{2}, x_{3}, x_{4}, x_{5}$ are randomly selected from the numbers $1, 2, 3, \ldots, 18$ and are arranged in increasing order $(x_{1} < x_{2} < x_{3} < x_{4} < x_{5})$. The probability that $x_{2} = 7$ and $x_{4} = 11$ is